In many applications, such as fuel injector nozzle tips, carburetor jets, cooling air flow through turbine engine components, lubricating oil metering for precision bearings and the like, metering of flow rates is of very great importance. However, due to manufacturing artifacts, it is of great difficulty. Even minute variations in manufacturing tolerances can produce substantial variations in flow resistance and flow.
Parts having fluid flow orifices are made by a wide variety of casting and machining procedures. For example, high quality investment castings are frequently employed for the manufacture of such parts. Even the high quality parts will have variations in dimensions, particularly wall thicknesses attributable to slight core misalignments or core shifting, and other variations in surface conditions, including surface roughness, pits, nicks, gouges, blow holes, or positive metal. In the extreme case, a very slight crack in a core can lead to a thin wall projecting into an internal passage. All these artifacts will substantially impede fluid flow.
Commonly employed machining methods, such as conventional drilling, electrical discharge machining and even less usual techniques as laser, electron beam and electrochemical techniques are not sufficiently precise to avoid the generation of substantial variations in flow resistance. Probably, the most precise of these, electrical discharge machining, will not produce perfectly uniform flow resistance because non-uniform EDM conditions are inevitable and may produce variations in size, shape, surface finish and hole edge conditions.
Such deviations are necessarily tolerated within broad limits and the attendant compromises in design freedom, performance and efficiency are accepted as unavoidable. For example, the delivery of fuel charges to internal combustion engines by pressurized fuel injection requires metering of flow through injector nozzles. The more precisely the flow can be regulated, the greater the fuel efficiency and economy of the engine operation.
At present, the design of such fuel injector nozzles is often based on the measurement of the actual flow resistance. The nozzles are segregated into different ranges of flow parameters to provide at least approximate matching of components within a range of deviation from defined tolerances. The inventory requirements for the matching of components is quite substantial and therefore very costly. In addition, a substantial number of components must be rejected as out of allowable deviations and must be reworked at considerable expense or discarded.
With diesel fuel injector nozzles, it has been found desirable to radius the inlet side of the injector microholes in order to eliminate stress risers and pre-radius the upstream edge to minimize changes in emissions over the design life of the nozzle. Conventional abrasive flow machining can effectively produce radii on microholes, but fine control of the final injector flow rate has been impossible to achieve. The high, putty-like viscosity and highly elastic character of conventional abrasive flow media are too radically different from the characteristics of diesel fuel to permit either in-process gauging or adaptive control of this process. Furthermore, the very small quantity of abrasive flow media required to produce the desired radius limits process resolution.
Briefly, in abrasive flow machining (AFM) of microholes the flow rate of the material does not correlate well to the flow rate of the target liquid. Therefore, the actual calibration of a microhole is a step-by-step fine tuning process. After radiusing and smoothing the microhole with AFM, the target liquid or calibration liquid is tested in the microhole, the microhole is further worked and the target liquid or calibration liquid is again tested, etcetera, until the target liquid tests correctly.
The present invention is based upon a statistically meaningful correlation between the flow rate of a liquid abrasive slurry through a microhole to a target liquid flow rate. When the abrasive liquid slurry reaches a predetermined flow rate the microhole is properly calibrated for the target liquid.
Liquid abrasive slurry flow as employed in the present application includes the flow of abrasives suspended or slurried in fluid media such as cutting fluids, honing fluids, and the like, which are distinct from semisolid polymer compositions. The liquid abrasive slurry of the invention is comprised of a liquid media, a Theological additive and abrasive particles. The abrasive particles remain uniformly distributed when the slurry is subjected to shear and the slurry decreases in viscosity when subjected to shear flowing through a microhole at a pressure of between 400 to 1000 psi.
The invention finds utility in the radiusing, polishing and smoothing of microholes in any workpiece, e.g. fuel injector nozzles, spinnerets. A liquid abrasive slurry flows through the microholes. The abrasive liquid flow rate correlates to the target flow rate of the liquid, for example diesel fuel, for which the fuel injector nozzle is designed. When the abrasive liquid slurry of the system reaches a predetermined flow rate the process is stopped. The microholes, without further iterative calibration steps, are properly calibrated for use with the target liquid, i.e. diesel fuel.
Although the preferred embodiment of the invention is described in reference to the radiusing, polishing and smoothing of microholes, it also includes the smoothing and polishing of non-circular apertures, i.e. rectangular slots, squares elliptical configurations, etc. The square area of the non-circular apertures would typically be less than approximately 3 mm.sup.2.
In a preferred embodiment, the invention is directed to radiusing and sizing the microholes in diesel fuel injectors using a liquid abrasive slurry with particular Theological properties. the abrading action at the inlet edge of the microhole results from the acceleration of slurry velocity as it enters the microhole. The radius produced and the finish imparted to the microhole is similar to that of abrasive flow machining. However, the relatively low slurry viscosity and its low abrasiveness at low velocity enables the use of a flow meter in the slurry flow path which can directly and accurately monitor slurry flow rate and slurry mass flow in real time. Therefore, tight process control is attained as compared with conventional abrasive flow machining. In the preferred embodiment of the invention, the slurry flow is correlated to diesel fuel flow rates. This allows for individual slurry processing of nozzles to their specified flow rates.
It is an object of the present invention to provide a method of radiusing and sizing microholes.
Another object is to provide a method for attaining a predetermined flow resistance through microholes with an abrasive liquid slurry having a flow rate which correlates to the flow rate of a target liquid.
A further object is to provide fuel injector nozzles having orifices with reproducible, precise, predetermined flow resistances.